devlink: fix return value check in devlink_dpipe_header_put()

Fix the return value check which testing the wrong variable
in devlink_dpipe_header_put().

Fixes: 1555d204 ("devlink: Support for pipeline debug (dpipe)")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/devlink.c

@@ -2031,7 +2031,7 @@ static int devlink_dpipe_header_put(struct sk_buff *skb,
 	int err;
 
 	header_attr = nla_nest_start(skb, DEVLINK_ATTR_DPIPE_HEADER);
-	if (!header)
+	if (!header_attr)
 		return -EMSGSIZE;
 
 	if (nla_put_string(skb, DEVLINK_ATTR_DPIPE_HEADER_NAME, header->name) ||